In the area of electronic distance measurement, various principles and methods are known. One approach consists in emitting frequency-modulated electromagnetic radiation, such as, for example light, to the target to be surveyed and then receiving one or more echoes from back-scattering objects, ideally exclusively from the target to be surveyed, it being possible for the target to be surveyed to have both a reflective—for example retroreflectors—and a diffuse back-scattering characteristic.
After reception, the optionally superposed echo signal is superposed with a mixed signal and the signal frequency to be analyzed is reduced thereby so that less outlay is required with respect to the apparatus. The mixing can be effected either as a homodyne method with the signal sent or as a heterodyne method with a periodic, in particular harmonic, signal of known period. Thus, the methods differ in that mixing is effected with the transmitted signal itself or with a harmonic signal having its own frequency. The mixing serves for transforming the received signal to lower frequencies and for amplifying said signal. Thereafter, the transit times and hence—in the case of a known propagation velocity of the radiation used—the distances to the targets to be surveyed are determined from the resulting signal. In a heterodyne interferometer arrangement, a tuneable laser light source is used for the absolute distance measurement. In the embodiment which is simplest in principle, the tuning of the optical frequency of the laser source is effected linearly. The received signal is superposed with a second signal which is derived from the emitted light signal. The resulting beat frequency of the heterodyne mixed product, the interferogram, is a measure of the distance to the target object. The apparatuses used for implementing these methods usually utilize a signal generator as a chirp generator, which impresses a signal on a modulatable radiation source. In the optical range, lasers which can be chirped by modulation of the external (for example Bragg grating) or internal cavity (for example Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR)) are generally used as radiation sources. In the optical range, transmitting and receiving optical systems to which a detector or quadrature detector for heterodyne mixing, A/D convertor and digital signal processor are connected down-circuit are used for emission and for reception.
An example of an optical, coherent FMCW distance-measuring method is described in U.S. Pat. No. 4,830,486, this method having an accuracy in the region of phase-measuring methods in combination with a short measuring time. A chirp generator produces a linear frequency-modulated signal which is divided into a measuring signal and local oscillator signal, the two signals being superposed in a receiver.